The present invention relates to a new and improved construction of an ionization type smoke detector having a single radioactive source.
The hitherto known ionization type smoke detectors may be classified into two categories. One such category encompasses two-radioactive source type smoke detector composed of an outer ionization chamber for measurement into which smoke can freely flow and an inner ionization chamber for comparison or reference into which no smoke can flow. Each of the ionization chambers is provided with a radioactive source. The other category encompasses single-radioactive-source type ionization smoke detectors in which an intermediate electrode partitioning the two ionization chambers has a small hole formed therein through which the outer ionization chamber is irradiated by a radioactive source disposed in the inner ionization chamber. The single-radioactive-source type smoke detector has advantages over the two-radioactive-source type smoke detector in that its structure is simplified and that a reduced amount of radioactive-energy is sufficient for operation of the smoke detector. However, operational tests of actual smoke detectors of the single-radioactive-source type have shown that there are disadvantages due to the structure which makes it difficult for smoke to flow into the inner ionization chamber through the small hole. Smoke which has once entered the outer ionization chamber is also difficult to be withdrawn therefrom and prevents the smoke detector from being rapidly restored to the initial or normal state. As an attempt to overcome the difficulty mentioned above, there has been made a proposal according to which the hole or opening formed in the intermediate electrode is enlarged in size while the ionization current flowing through the inner ionization chamber is maintained in a saturated state so that the resistance value of the inner ionization chamber remains invariable independent of smoke flowing therein. In this structure of the single-radioactive-source smoke detector, however, the radioactive source disposed within the inner ionization chamber is constantly exposed to the atmosphere, resulting in the radioactive source being subjected to contamination which, in turn, gives rise to changes in the ionization current and hence erroneous operation as well as generation of false alarms.